Some diagnostic or interventional medical procedures require implanting one or more leads through a hole in a patient's cranium. Once the surgeon places a lead so that the distal end is at a desired location, the surgeon wants the lead to stay there for whatever ultimate medical purpose the lead has, for example, to monitor physiological parameters from a patient or to deliver a therapy to the patient. Usually, however, there are steps subsequent to placing the lead that are necessary to complete the surgical procedure, and some of these may cause inadvertent displacement of the lead away from the desired location, which may not be corrected easily or efficiently.
Generally, leads may be provided with electrodes configured to sense information from the brain or to deliver a form of stimulation to the brain intended to modulate neural activity, such as electrical stimulation. The sensing and/or stimulation may occur at a distal end of the lead, for example, through electrodes exposed to brain tissue at a distal end, wherein the signals are communicated through conductors disposed in the lead body extending to a lead proximal end. Connections available at the lead proximal end allow the lead to be connected to another medical device, implanted or external, that processes the sensed signals and/or generates the form of stimulation.
For example, in the responsive neurostimulation system manufactured under the tradename INNS SYSTEM by NeuroPace, Inc., the proximal ends of one or more implanted brain leads can be connected to another implanted medical device, namely a neurostimulator that is seated in a tray or ferrule in a craniectomy in the patient's skull. In another example, the leads may be implanted in a patient so that an intracranial monitoring procedure can be undertaken for a period (e.g., several days or a couple of weeks), with the proximal ends of the implanted leads being connected to external equipment monitoring the patient's brain activity, such as to identify a focus or the foci of epileptiform activity in the patient.
There are multiple types of brain leads currently available. In applications where the leads are being used to sense or stimulate brain tissue at or near the focus of undesirable epileptiform activity, there is a depth lead (also sometimes referred to as a “deep brain lead” or as a “stereotactic depth lead”, because this lead type is often implanted using stereotaxy, a three-dimensional localization and placement procedure) and a cortical strip lead (also known simply as a “cortical lead” or as a “subdural lead”, because this lead type is usually implanted underneath the dura mater).
A depth lead is implanted so that the distal end is located in the brain tissue, in or adjacent a structure that is deemed to be associated with the generation of the undesirable activity. A cortical strip lead is implanted so that the distal end lays on a surface of the brain at or adjacent brain tissue that is believed to comprise an epileptic focus. The intended location of the distal end of the brain lead in or on the brain is referred to hereinafter as the “target”.
A lead manufacturer may make different lead types in one or more standard lengths, rather than in lengths customized for a particular application in a particular patient. In addition, the depth lead type may be intended to be implanted using stereotactic equipment or some other tool that requires some length in excess of that which is needed to extend from the cranium and the connection to another medical device and the target (e.g., so that the lead is long enough to extend through the distance required when using a stereotactic frame mounted to the patient). For at least the reason that a lead may be manufactured to have more length than is necessary to traverse the distance between the proximal connection to another implant or external equipment and the target, brain leads are often manufactured to be quite flexible. That is, if the lead is flexible, excess length can be coiled or folded at the surface of the skull before the scalp is replaced. Further, flexibility may be considered a better alternative than a stiff lead when the lead is to remain in place in or on a surface of the brain chronically, as opposed to acutely, such as to minimize tissue damage and to optimize the integrity or resolution of signals. Brain leads manufactured and sold with the RNS SYSTEM, for example, have a flexibility on par with that of a piece of cooked spaghetti.
When implanting a lead in the brain tissue, though, its relative flexibility can present challenges when delivering the distal end to the target. Accordingly, a brain lead is often provided with an inner lumen through which a removable stiffener, such as a stylet, can be disposed. The stylet lends stability to the lead while the distal end is routed to the target, and is then removed when the lead has been positioned where it is intended to remain, either acutely or chronically. It is undesirable, however, if the act of withdrawing the stylet causes the distal end to move away from the target.
Equipment or tools used in implanting a brain lead can also unintentionally cause the distal end of a brain lead to move away from the target in procedural steps undertaken subsequent to placing the lead. For example, a slotted cannula is often employed in implanting leads stereotactically, in which a hole is formed in the patient's skull at a location calculated to allow an appropriate trajectory of a lead to deep brain target. In one example of such a procedure, a frame is attached to the patient, and a guide tube is oriented to achieve the desired trajectory relative to the skull hole. A cannula is inserted through the guide tube, such that its range of motion is constrained by the guide tube. The cannula may be provided with a removable rod disposed in an inner lumen thereof. The cannula is advanced through the skull hole towards the target. The inner rod in the cannula prevents tissue from backing up into the cannula while it is advanced.
When the cannula has been advanced to or approximate the target, the inner rod is removed and a depth lead inserted into the cannula lumen. The surgeon then advances the depth lead to the target. (Sometimes the depth lead is marked in advance at a proximal location, for example, with a stop gauge, to provide feedback to the surgeon when the target has been reached.) Once the distal end of the lead reaches the target, the cannula must be withdrawn from the brain and the lead must be extracted from the cannula, so the stereotactic equipment can be removed. A cannula is often provided with a longitudinally-extending slot with a width wide enough to accommodate the diameter of the lead body for this purpose, i.e., so that the lead can be stripped away from the cannula using the slot, rather than having to retract the cannula over the lead body. This allows the lead to be manufactured with somewhat less excess lead length than if the lead had to be long enough to retract the cannula over the very proximal end of the lead. As is the case with withdrawing a stylet, it is undesirable if the act of disengaging the lead body from a cannula (or other apparatus used for stereotaxy) causes the distal end of the brain lead to move away from the target.
A hole in the skull is often formed with some standard diameter, owing to the drills typically available in the operating room to create it. When an air drill is used to create a hole in the skull with a diameter of 5 mm or greater, the skull hole is often referred to as a “burr hole.” Surgeons create standard-sized burr holes, because there are surgical accessories intended for use with burr holes that are intended for use with certain burr hole diameters, such as 14 mm. However, the diameter of a brain lead may be much smaller than that of a burr hole, because 14 mm is on the order of ten times greater than the diameter of the lead to be implanted. For example, some brain leads manufactured by NeuroPace, Inc. have a diameter of only 1.27 mm. Therefore, in some cases a surgeon may choose to use a smaller diameter hole through which to implant a lead. For example, a surgeon may choose to use a hand-held twist drill to create a hole with a diameter on the order of less than 5 mm (depending on the diameter of the twist drill bit: a common one results in a 3.2 mm diameter hole). A skull hole formed using a twist drill is sometimes referred to as a “twist drill hole”.
In view of the foregoing, it would be beneficial to provide accessories and tools that reduce the likelihood that the distal end of a brain lead will be dislodged after it has been implanted at a target, as a consequence either of surgical steps or post-surgical factors, such as a patient fiddling with the lead or the skull hole through the scalp. Embodiments of a lead fixation accessory, a lead stabilization tool, and method of using them disclosed herein address these needs and others.